Target and Amazon.com were pulling out all the stops, offering free delivery with no minimum order requirement and bombarding shoppers with promotional emails.
Companies' sales were up 20 percent from a year earlier as of 7 p.m. ET, including a surge in orders placed via mobile phones, Adobe Analytics found.
On Wall Street, investors showed their enthusiasm. Shares of Amazon closed up 5 percent. Macy's, Kohl's, and Target shares rose as well.
In another estimate, Mastercard SpendingPulse forecast a 25 percent jump in e-commerce sales to at least $3 billion, based on sales via the Mastercard payments network and estimates for other payment forms such as cash and check.
Amazon on Tuesday said Cyber Monday was once again the single biggest shopping day in the company's history. The so-called Turkey 5 — the five popular shopping days starting with Thanksgiving and continuing through Cyber Monday — broke records, as well. Amazon said customers ordered more than 18 million toys and more than 13 million fashion items on Black Friday and Cyber Monday, combined.
These numbers still paled in comparison to Alibaba's "Singles Day" earlier this month, when the Chinese e-commerce giant raked in $30.7 billion in sales.
Some industry observers saw a down side to the U.S. shopping frenzy.
Bob Phibbs, chief executive of New York-based consultancy the Retail Doctor, warned of potential store closures if brick-and-mortar players discounted items too heavily this holiday season.
"Online discounts and free shipping are significantly cutting into retailers' profits," he said. "Retailers are just spending money in the hopes that they don't lose too much more."
The promotional efforts also drew the ire of customers who complained they woke up to even more Cyber Monday emails than in years past.
"Yes retailers, I'm aware it's Cyber Monday even without the 150 emails," tweeted Keina (zRealMamaEagle), a user from Delaware.